The present invention is related to the field of printing and in particular to a method of coating substrates to improve adhesion of toner images thereto.
Problems of transfer and adhesion of toner images and in particular of liquid toner images, to substrates, is well known. Transfer of images may be effected by electrostatic transfer of toner particles to the substrate, either from an imaging surface such as a photoreceptor or from an image transfer member. Alternatively, image transfer may be effected by heat and pressure, generally from an intermediate transfer member to which the image is first transferred from the imaging surface. Image transfer from an intermediate transfer member, on which the image is generally in a molten state, is often problematic. Usually, some treatment of the substrate surface is required in order to enable good transfer and fixing.
Transfer and adhesion of toner is improved by chemical compatibility of the toner and the substrate for example, chemical bonding by acid-base interaction or chemical similarity leading to mutual bonding.
A common solution to transfer and adhesion problems is the coating of a substrate with an adhesion enhancer, which has an affinity for both the toner material and the substrate. For example, the adhesion of the above mentioned toner types to PET and BOPP can be greatly enhanced by coating the substrates with a layer of Macromelt 6239 (Henkel) which is an amine terminated polyamide. This material adheres well to the substrate and has a high affinity for the toner particle polymer as well. However, it has been found that the shelf life of such coated substrates is limited and the quality of the transfer of toner from an intermediate transfer member to the substrate degrades with time. This aging process of the coating does not seem to effect the adhesion of images which are already transferred to the substrate.
However, as is well known in the art, such substrate processing does not always lead to satisfactory results, especially in digital electrostatic printing.
EP Patent Publication EP 0 789 281, the disclosure of which is incorporated herein by reference, describes a coating system for polypropylene in which the polypropylene is coated with polyethylene and then with ethylene-acrylic acid and minor amounts of filler such as talc or silica and wax and/or pigment.
EP Patent Publication EP 048 481 describes a coating system for coating a polymeric substrate with a first lacquer layer, preferably containing a particulate material. The lacquer layer is overcoated with a toner receptive layer containing a copolymer comprising styrene and/or a styrene derivative and at least one ethylenically unsaturated co-monomer co-polymeriserable therewith, the copolymer containing at least one free functional acid group. The toner preferably comprises an additional acrylic and/or metacrylic polymer to improve the ink receptivity of the layer. Optionally, an adhesion promoting layer may be situated between the lacquer layer and the polymeric substrate.
US Pat. No. 5,126,198 describes a coated substrate that is not described as being suitable for forming toner images thereon. Rather this material is used as a heat laminatable multi-layer film. The outer, heat laminatable layer comprises a mixture of polymers.
One aspect of some preferred embodiments of the present invention is a novel approach to the improvement of transferability and/or adhesion of images to substrates, especially plastic substrates.
In a broad view of some preferred embodiments of the invention, a double coating is employed, with an underlayer having a high affinity for the substrate and an overlayer having a high affinity for the toner material. The two layers have a high affinity for each other.
In an exemplary embodiment of this aspect of the invention, the underlayer is a layer of an amine terminated polyamide such as Macromelt 6239 (Henkel) which was mentioned in the background of the invention. The upper layer is formed of material which has a high affinity for toner particles and which is also adheres well to the underlayer. As indicated above, an amine terminated polyamide has a high affinity for a wide rage of materials so that this seldom is a problem. Furthermore, the above mentioned deterioration of the properties of the amine terminated polyamide with time does not exist when it is coated with the overlayer.
Exemplary materials which are useful as overlayers include ethylene acrylic acid copolymer, polyvinyl pyridine and styrene butadiene copolymer. Preferably, the overlayer consists substantially only of these materials and more preferably consists substantially only of one of these materials. It is noted that these materials adhere rather poorly to untreated PET, BOPP, polycarbonate, polyethylene and vinyl. For some applications it is useful to reduce the degree of acidity of the ethylene acrylic acid copolymer, preferably by Saponification to a level of preferably between 8 and 18% acrylic acid comonomer weight percent, more preferably between 10 and 16%. Unmodified material typically has a acrylic acid comonomer weight percent of 20%. In preferred embodiments of the invention, the overlayers are free of silica, talk or other such fillers and free of wax or pigments.
A second aspect of some preferred embodiments of the invention relates to substrates which are coated with ethylene acrylic acid copolymer, polyvinyl pyridine or styrene butadiene copolymer, and especially to substrates coated with ethylene acrylic acid copolymer with reduced acidity. Such coated substrates may, in some preferred embodiments of the invention, be plastic substrates such as PET or BOPP or, in other embodiments of the invention, may be paper. Preferably, the overlayer consists substantially only of these materials and more preferably consists substantially only of one of these materials.
A third aspect of some preferred embodiments of the invention relates to the printing of images, preferably toner images and more preferably liquid toner images, on substrates of the invention or which are produced in accordance with the invention. This printing is preferably by transfer of the toner images to the substrate from an intermediate transfer member, to which it was transferred from an imaging surface. Preferred toners are based on one or more of ethylene vinyl acetate (EVA) copolymers, copolymers of ethylene and xcex1, xcex2-ethyleneically unsaturated acid selected from the group consisting of acrylic and methacrylic acids and ionomers such as are produced under the trade name of Surlyn, by Dupont.
There is therefore provided in accordance with a preferred embodiment of the invention, a substrate suitable for printing a toner image thereon, comprising:
a sheet of plastic;
an underlayer coating, on the sheet of plastic, comprising a first polymer material comprising a polymer chosen from the group consisting of amine terminated polyamide, a silane coupling agent and amino propyl triethoxy silane;
an overlayer coating, directly on the underlayer, comprising a second polymer material and having an outer surface to which a toner image can be fused and fixed.
There is further provided, in accordance with a preferred embodiment of the invention a substrate suitable for printing a toner image thereon, comprising:
a sheet of plastic;
an underlayer coating, on the sheet of plastic, comprising a first polymer material;
an overlayer coating, directly on the underlayer, comprising a second polymer material and having an outer surface to which a toner image can be fused and fixed, the second polymer consisting essentially of a polymer chosen from the group consisting of ethylene acrylic acid copolymer, polyvinyl pyridine and styrene butadiene copolymer.
Preferably, the first polymer material comprises a polymer chosen from the group consisting of amine terminated polyamide, a silane coupling agent and amino propyl triethoxy silane.
Preferably, the sheet of plastic is PET. Alternatively, the sheet of plastic is BOPP. Alternatively, the sheet of plastic is polyethylene. Alternatively, the sheet of plastic is vinyl. Alternatively, the sheet of plastic is polycarbonate.
In a preferred embodiment of the invention, the overlayer comprises ethylene acrylic acid copolymer. Preferably, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of less than 18%. Alternatively, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of less than 16%.
Alternatively or additionally, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of more than 8%. Alternatively, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of more than 12%.
In a preferred embodiment of the invention, the overlayer comprises polyvinyl pyridine. Alternatively, the overlayer comprises styrene butadiene copolymer.
In a preferred embodiment of the invention, the underlayer comprises amine terminated polyamide. Alternatively, the underlayer comprises a silane coupling agent. Alternatively, the underlayer comprises amino propyl triethoxy silane.
In a preferred embodiment of the invention, the underlayer has a weight of between 0.1 and 1 grams per square meter. Alternatively or additionally, the underlayer has a weight of between about 0.3 and 0.5 grams per square meter.
In a preferred embodiment of the invention, the overlayer has a weight of between 0.1 and 10 grams per square meter. Alternatively or additionally, the overlayer has a weight of between 0.2 and 2 grams per square meter. Preferably, the overlayer has a weight of between about 0.25 and about 0.35 grams per square meter.
Preferably, the overlayer is substantially wax and pigment free.
Preferably, the overlayer is substantially free of particulate matter. Preferably, the underlayer is substantially free of particulate matter. More preferably, both the overlayer and the underlayer are free of particulate matter.
Preferably, the substrate comprises only two coating layers.
There is further provided in accordance with a preferred embodiment of the invention, a method of producing a coated substrate which a toner image can be adhered comprising:
coating a sheet of plastic with a first polymer material as an underlayer, the underlayer comprising a polymer chosen from the group consisting of amine terminated polyamide, a silane coupling agent and amino propyl triethoxy silane;
directly overcoating the underlayer with an second polymer material to form an overlayer coating on the underlayer, the overlayer having an outer surface to which a toner image can be adhered and fixed.
There is further provided, in accordance with a preferred embodiment of the invention, a method of producing a coated substrate which a toner image can be adhered comprising:
coating a sheet of plastic with a first polymer material as an underlayer;
directly overcoating the underlayer with an second polymer material to form an overlayer coating on the underlayer, the overlayer having an outer surface to which a toner image can be adhered and fixed, the second polymer consisting essentially of a polymer chosen from the group consisting of ethylene acrylic acid copolymer, polyvinyl pyridine and styrene butadiene copolymer.
Preferably, the first polymer material comprises a polymer chosen from the group consisting of amine terminated polyamide, a silane coupling agent and amino propyl triethoxy silane.
There is also provided in accordance with a preferred embodiment of the invention, a substrate comprising a sheet chosen from the group consisting of PET, BOPP, polycarbonate, polyethylene and vinyl and an outer coating consisting substantially only of a polymer chosen from the group consisting of ethylene acrylic acid copolymer, polyvinyl pyridine and styrene butadiene copolymer.
Preferably, the sheet is a PET sheet. Alternatively, the sheet is a BOPP sheet. Alternatively, the sheet is a polycarbonate sheet. Alternatively, the sheet is a polyethylene sheet. Alternatively, the sheet is a vinyl sheet.
Preferably, the coating comprises polyvinyl pyridine. Alternatively, the coating comprises styrene butadiene copolymer. Alternatively, the coating comprises ethylene acrylic acid copolymer. Preferably, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of less than 18%.
There is also provided in accordance with a preferred embodiment of the invention, a substrate comprising a sheet and a coating of ethylene acrylic acid copolymer having an acrylic acid comonomer percentage weight of less than 18%.
Preferably, an ethylene acrylic acid copolymer, as described above, has an acrylic acid comonomer percentage weight of less than 16%. Alternatively, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of more than 8%.
Preferably, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of more than 12%.
Preferably, the coating, as described above, has a weight of between 0.1 and 0.4 grams per square meter. Preferably, the coating has a weight of between about 0.25 and about 0.35 grams per square meter.
There is also provided in accordance with a preferred embodiment of the invention, a printing method comprising:
providing a substrate, as described above; and
printing a toner image on the substrate.
Preferably, the toner image is a liquid toner image. Alternatively or additionally, printing comprises transferring the toner image to the substrate using heat and pressure. Alternatively, printing comprises electrostatically transferring the toner image to the substrate.
In a preferred embodiment of the invention, the printing method comprises:
forming the image on an image forming surface;
transferring the image from the image forming surface to an intermediate transfer member, and transferring the image from the intermediate transfer member to the substrate.